The present invention relates to the detection of submerged objects in a scattering medium such as water. More specifically, but without limitation thereto, the present invention relates to a device for forming high resolution images of objects submerged in shallow water and coastal regions from an airborne platform using improved lidar (light detection and ranging, analogous to radar, i.e., radio detection and ranging) to provide high resolution imaging.
A number of military and civilian applications require searching for certain objects in a scattering medium. For example, moored and bottom mines deployed in shipping lanes generally must be detected before measures can be taken to disarm them. It is also useful in various applications to locate and map submerged obstacles, cables, pipelines, barrels, oil drums, etc.
An imaging lidar is commonly applied to the problem of detecting submerged objects in shallow water, such as mines. An exemplary lidar is described in U.S. Pat. No. 5,243,541 issued to Ulich on Sep. 7, 1993 incorporated herein by reference thereto. This lidar improves the spatial resolution of objects by pulsing the laser and range gating the photodetector to exclude scattered light from the surface and depths not of interest.
Line scanning is another image acquisition technique that is typically used with a laser on a moving submerged platform. The laser scans the ocean bottom transversely with respect to the direction of motion of the platform and images the scattered light with a narrow field of view photomultiplier tube. In order to generate an image at a practical resolution, the scan rate should be about 700,000 pixels per second. A slower scan rate would increase the data acquisition time, causing vulnerability in hostile environments, or reduce the image resolution.
A problem with current scanning lidars is that they perform poorly in ambient light. Because blue-green lasers are typically used for underwater transmission, sunlight scattered back to the photomultiplier tube degrades the signal to noise ratio. Another problem is that surface scattering dictates that the laser/detector platform be submerged to prevent heavy losses in the transmitted signal. Still another problem is that a 700 KHz scan rate dictates the use of CW lasers, because most lasers cannot be pulsed at rates on the order of 700 KHz without significantly degrading the laser efficiency, which prevents locating the laser/detector platform on an aircraft for use above the water surface.